Organic Chemistry 2 · Addition to aldehydes and ketones
Bisulfite adduct of heptanal: 1-hydroxyheptane-1-sulfonate
Conditions: NaHSO3, H2O
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The mechanism, step by step
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Sulfur of bisulfite attacks the aldehyde
The bisulfite ion is a soft, strong nucleophile at sulfur, which carries a lone pair. Sulfur bonds to the carbonyl carbon of heptanal while the C=O π pair moves onto oxygen, and the O– on sulfur forms a second S=O bond, giving a sulfonic acid group.
The curved arrows
- The sulfur lone pair forms the new C–S bond to the aldehyde carbon.
- The C=O π pair moves onto oxygen, which becomes an alkoxide.
- A lone pair of the negatively charged oxygen on sulfur forms a new S=O π bond.
Water removes the sulfonic acid proton
The SO3H group is a strong acid, so water removes its proton easily and forms hydronium. The sulfonate anion that remains is stabilised over three oxygens.
The curved arrows
- A water lone pair takes the acidic sulfonic acid proton.
- The O–H pair stays on oxygen as the sulfonate.
Hydronium protonates the alkoxide
The alkoxide is a strong base and takes the proton from hydronium. The net result is that the bisulfite proton has moved from the sulfur group to the former carbonyl oxygen.
The curved arrows
- The alkoxide lone pair picks up the proton from hydronium.
- The O–H pair stays on oxygen, releasing water.
Sodium 1-hydroxyheptane-1-sulfonate
The bisulfite adduct is an ionic sodium salt that crystallises or dissolves in water, while non-aldehyde impurities stay in the organic layer. Treating it with aqueous base later gives back pure heptanal.
What you'll practice
Draw the sulfur of bisulfite adding to the carbonyl carbon of heptanal and the water-relayed proton transfer that gives sodium 1-hydroxyheptane-1-sulfonate.
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